The present specification generally relates to detecting marks on disk surfaces for servo signals.
An optical disk drive uses a laser beam to write and read data on a storage media. The disk drive uses the same laser beam to read the servo signals, including a tracking error signal (TES) and read-only (RO) signals. The TES may be generated by detecting a diffraction pattern of light from pre-formatted areas, e.g. grooves on a disk surface. The TES provides information about the laser beam position on a grooved disk. The TES is fed into a control system to keep the illuminated beam aligned relative to the center of a selected track. It is desirable to keep the beam on the center if the data is recorded on the center of the track.
The RO signals mainly provide address information for the disk surface, such as a sector number and a track number. The RO signals are encoded in the pre-formatted marks on the disk surface. Since the reflectivity of the pre-formatted mark is different from the reflectivity of the flat region, pre-formatted marks can be detected by measuring a sum signal, which is the total amount of light reflected from the disk surface, and then by electronically performing differentiation of the sum signal. If the sum signal is below (or above) a predetermined level, it indicates that a focused spot of the laser beam is crossing a mark. If the sum signal is above (or below) the predetermined amount, then it indicates the focused spot being on a flat area. The encoded RO signals are decoded by successively measuring the sum signal as the laser beam scans over a series of pre-formatted marks.
A near-field optical disk drive operates by placing the optical head near the storage media by a spacing less than about one wavelength (e.g., a fraction of one wavelength). Unlike coupling in a far-field configuration based on light propagation, optical signals in the near-field configuration are coupled between the optical head and the storage media at least in part by the evanescent fields. Hence, each optical signal coupled through the optical head has a dependence on the air gap between the head and the media. Any variation in the air gap may cause a variation in an optical signal which acts as noise in signal detection.
The present disclosure provides an edge detection technique to generate a RO signal that is insensitive to the air gap spacing and the structure of the film stack in the media. The RO signal generated by this edge detection technique mainly changes with the edge geometry of the pre-formatted marks on the media and is decoupled from other optical signals including the data-bearing signal (e.g., the magneto-optic signal or the phase-change signal) and the TES signal. Hence, the structure of the media can be optimized for data recording and reading without significantly compromising generation of servo signals.
The inventor noticed that accurate timing of the pre-formatted mark edges can be detected, preferably without performing any electronic differentiation of the total reflected energy, by simply measuring the spatial energy distribution of the reflected beam. The pre-formatted mark edges can be accurately detected even under the high variation of the head| media spacing often encountered in near-field readout situation by monitoring the energy difference between the two halves of the reflected laser beam. Since the energy distributions in the two halves vary together with the signal variations caused by the head-media spacing, the film structure, and other common-mode factors, the energy difference between the two halves changes primarily with the geometry of the media surface changes and rejects common-mode noise.
In one aspect, the present specification involves an edge detection system for detecting pre-formatted marks on a disk surface by analyzing the energy distribution of a laser beam reflected from the disk surface. In some embodiments, analysis of the energy distribution includes receiving the laser beam and optically differentiating the energy from the two halves separated by a line in a selected direction to generate an edge detecting read-only (RO) signal.
In another aspect, a disk drive system including an edge detection system is disclosed. One embodiment of the disk drive system includes an optical head that couples the laser beam onto the surface of the storage media. The system also includes a servo detector have two sensing areas to detect energy distributions from two different portions of the laser beam and an edge detection analyzer that computes the difference between the portions.